Japan's Foreign Ministry said in a statement it filed the protest after Japan's coast guard spotted the vessels Saturday along with a fleet of 230 Chinese fishing boats swarming around the Japanese-controlled Senkaku Islands. China also claims the islands, calling them the Diaoyu.

A group of disputed islands, Uotsuri island (top), Minamikojima (bottom) and Kitakojima, known as Senkaku in Japan and Diaoyu in China is seen in the East China Sea. Reuters

Japan is demanding the ships leave the area. Three of the Chinese coast guard vessels were armed with what appeared to be gun batteries, Japan's Foreign Ministry said.

The Chinese fleet has not intruded in Japanese territorial waters so far, it said.

China's increasingly assertive maritime activities in the East and South China seas have raised concerns and tensions in the region. Japan has joined the U.S., the Philippines and others in urging China to abide by international law after a U.N. arbitration panel ruled in favor of the Philippines in its maritime dispute with Beijing in the South China Sea.

China says it doesn't recognize the 12 July ruling by the Permanent Court of Arbitration.

Relations between Japan and China have been strained over wartime history, a sensitive topic for the region in times of war-end anniversaries during the summer.

China's coast guard vessels routinely sail around the islands, usually in pairs or up to four. Until now, only one of the vessels was armed.

Saturday's fleet size and equipment showed "an escalation of the situation that could heighten tensions in the waters," the ministry said, without elaborating on what might have caused the increase.

On Friday, Japan also protested after two Chinese coast guard ships entered the Japanese-claimed waters around Senkaku. There has been no immediate comment from Beijing.